Abstract: The mechanism of the concrete filled square steel tubular beam-columns strengthened by CFRP externally ( S-CF-CFRP-ST) were analyzed by using ABAQUS software，and the results showed that the compressive region on cross-section varied from compression on the whole cross-section to tension on partial cross-section with loading increase． The steel tube yielded firstly on the cross-section at mid-height，and then the yielding propagated toward the end plates． The transverse CFRP in tension provided effective confinement on the specimens． The interaction force between the steel tube and the concrete reached its maximum value on the cross-section at mid-height，and it decreases when the distance to the end plates became smaller． On the cross-section，the interaction force had its maximum value at the corner while it decreased when the position moved to the mid-point of the tube side． The adhesive strength between the steel tube and the concrete had little effect on the load carrying capacity，and on the elastic stiffness of the specimens as well as on the interaction force between the steel tube and the concrete． The static behavior of the members was not sensitive to the different loading routes． Equation for calculating the loading carrying capacity of the composite columns was presented，and the estimated results agreed well with the experimental results．